This invention relates to an electric cell that converts the chemical energy obtained in a fuel oxidation reaction directly into electric energy in a continuous process. More specifically the invention relates to fuel cells.
Fuel cells are often described as continuously operating batteries or as electrochemical engines. Fuel cells utilize an external supply of fuel and oxygen (or air) and produce power continuously, as long as the fuel and oxygen supply is maintained.
The most classic fuel cell is the H2/O2 fuel cell of the direct or indirect type, wherein hydrogen is oxidized to form H3O+at the anode and oxygen is reduced to water at the cathode. In the direct type, hydrogen and oxygen are used as such, the fuel being produced in independent installations. The indirect type employs a hydrogen-generating unit which can use as raw material a wide variety of fuels.
Another type of fuel cell is the organic fuel cell. In a direct oxidation cell an aqueous solution of an organic fuel such as methanol, formaldehyde or formic acid, is directly fed into the fuel cell without any previous chemical modification, where the fuel is oxidized at the anode, and oxygen is reduced to water at the cathode.
A major distinguishing characteristic of different fuel cells is in the electrolyte used. NASA""s Jet Prepulsion Laboratory (JPL) developed a direct liquid-feed cell using a solid membrane electrolyte. A detailed description of JPL""s fuel cells can be found, for example, in U.S. Pat. Nos. 5,599,638 and 5,773,162. These fuel cells operate without any acid electrolyte and comprise solid electrolyte membranes fabricated from proton-exchange materials, especially Nafion(trademark) (manufactured by DuPont). When methanol is used as the fuel, the electro-oxidation of methanol at the anode can be represented by:
CH3OH+H2Oxe2x86x92CO2+6H++6e,
and the electro-reduction of oxygen at the cathode can be represented by:
O2+4H++4exe2x86x922H2O.
Protons generated at the anode are transported directly across the electrolyte membrane to the cathode. A flow of current is sustained by a flow of ions through the cell and electrons through the external load.
JPL""s fuel cells and other related fuel cells known in the art, which use Nafion as solid electrolyte membrane, suffer from several drawbacks resulting from such membrane, among them high cost, high permeability of the membrane to the fuel (high fuel crossover), sensitivity to heavy metals impurities, lack of operation at temperatures below 0xc2x0 C. or over 100xc2x0 C. and high sensitivity to water management in the membrane.
The challenge in fuel cell development for practical applications is to improve the economics through the use of low-cost components with acceptable life and performance. In view of the above-mentioned problems associated with solid electrolyte membranes such as Nafion and related materials, it is an object of the present invention to provide a novel, low cost fuel cell having a reduced fuel crossover, reduced sensitivity to metal ion impurities and ability to operate at temperatures even higher than 100xc2x0 C. or lower than 0xc2x0 C. It is a further object of the invention to provide new effective organic fuels which undergo clean and efficient oxidation in the fuel cell. It is yet a further object of the invention to provide improved methods for catalyst preparation. Still, it is an object of the present invention to provide a new integrated flow field system for circulating gas and electrolyte in a H2/O2 fuel cell.
Other objects of the invention will become apparent as the description of the invention proceeds.
Thus, the present invention provides by the first of its aspects a fuel cell comprising a housing, a solid electrolyte membrane having a first and a second surface, disposed in said housing to partition it into an anode side and a cathode side, an anode and a cathode each comprising a catalyst layer and a carbon backing layer and being formed on said first and second membrane surfaces respectively, so as to connect said first surface to the anode side and said second surface to the cathode side, said anode side comprising means for storing an oxidizable fuel or for circulating it past the anode and said cathode side comprising means for flowing oxygen or air past the cathode, wherein said solid electrolyte membrane is a proton conducting membrane having pores with a diameter size which is essentially smaller than 30 nm and comprising:
(i) 5% to 60% by volume of an electrically nonconductive inorganic powder having a good acid absorption capacity, said powder comprising essentially nanosize particles;
(ii) 5% to 50% by volume of a polymeric binder that is chemically compatible with acid, oxygen and said fuel; and
(iii) 10 to 90% by volume of an acid or aqueous acid solution.
The solid proton conducting membrane used in the fuel cells of the present invention has been described in WO 99/44245. The polymeric binders used in these membranes are selected from the group consisting of poly(vinilydenfluoride), poly(vinilydenfluoride)hexafluoropropylene, poly(tetrafluoroethylene), poly(methyl methacrylate), poly(sulfonamide), poly(acrylamide), poly(vinylchloride), acrylonitrile, poly(vinylfluoride), Kel F(trademark) and any combinations thereof.
The inorganic nanosize powder used for preparing the solid proton conducting membrane is selected from the group consisting of SiO2, ZrO2, B2O3, TiO2, Al2O3, hydroxides and oxy-hydroxydes of Ti, Al, B and Zr, and any combinations thereof.
The proton conducting membrane used in the fuel cell of the invention also comprises an acid. As opposed to the solid electrolyte membrane described for example in U.S. Pat. No. 5,599,638, wherein no acid is present in free form, the solid electrolyte membrane used in the fuel cell of present invention contains free acid molecules entrapped in the pores of the membrane. Alternatively, it may contain acid molecules bonded to the inorganic powder. The typical diameter of these pores is essentially smaller than 30 nm, preferably smaller than 20 nm, more preferably smaller than 3 nm.
A large variety of low vapor pressure acids that are compatible with the cell hardware and with the catalysts at both electrodes can be used and adapted to a specific application. The following list of acids is given for example: polyfluoroolefin sulfonic acid, perfluoroolefin sulfonic acid, polyfluoroaryl sulfonic acids such as polyfluorobenzen, polyfluorotoluene, or polyfluorostyrene sulfonic acid, perfluoroaryl sulfonic acids such as perfluorobenzene, perfluorotoluene or perfluorostyrene sulfonic acid, similar acids where up to 50% of the hydrogen or fluorine atoms were replaced by chlorine atoms, CF3(CF2)nSO3H, HO3S(CF2CH2)nSO3H, CF3(CF2CH2)nSO3H, HO3S(CF2)nSO3H where n is an integer having a value of 1 to 9, Nafion(trademark) ionomers, HCl, HBr, phosphoric acid, sulfuric acid and mixtures thereof HCl and HBr are not recommended for use in combination with noble metal catalysts like Pt.
The anode and the cathode comprise a catalyst layer and a porous backing layer. A preferred catalyst used at the anode is for example nano size platinum-ruthenium powder, while preferred catalysts used at the cathode are for example nano size platinum powder and alloys thereof with non noble metals, for example Ni, Co, and Fe. In such alloys the ratio between platinum and the metal (Pt:M atomic ratio) is between about 1:3 to about 3:1.
The backing layer is preferably made of carbon. This layer is porous and is used for support and at the same time for making electrical contact between the housing and the catalyst powder, which by itself is connected to the membrane.
The means for circulating an oxidizable fuel past the anode and for flowing oxygen or air past the cathode include also means for withdrawing carbon dioxide, unused fuel and water from the anode side and for withdrawing unused oxygen and water from the cathode side.
The present invention also provides a new integrated system for circulating gas and electrolyte in a H2/O2 fuel cell. According to such a system the proton conducting membrane does not dry out, the acid is not washed away and the cell can operate at high temperatures.
A typical oxidizable fuel used in the invention is methanol. However, the present invention also provides new effective organic fuels which undergo clean and efficient oxidation in the fuel cell. These fuels are organic molecules having either one carbon atom or when having two or more carbons, then there are no two adjacent alkyl groups. In addition, all carbons except for methyl groups, are partially oxidized. Examples of new fuels are glycerol, ethanol, isopropyl alcohol, ethylene glycol and formic and oxalic esters thereof, oxalic acid, glyoxilic acid and methyl esters thereof, glyoxylic aldehyde, methyl format and dimethyl oxalat.